Hayward H-Series HS Error Code: High Limit Switch & Temperature Sensing Errors

Understanding the HS Code

The HS error code appears when the high limit switch detects water temperature exceeding 135°F in the heat exchanger. This is a critical safety feature that prevents damage to the copper heat exchanger and protects against scalding water temperatures.

The high limit switch is a normally closed device that opens when temperature exceeds its setpoint. Once tripped, it must cool down approximately 30°F before it automatically resets. The heater will not operate until the switch closes again.

High Limit System Components

The temperature safety system includes:

• High limit switch: Fixed temperature switch set at 135°F
• Water temperature sensor: Provides actual temperature reading to control
• Thermal well: Dry pocket where sensors mount in water path
• Control board: Monitors both devices and controls operation

Common Causes of HS Codes

Flow-Related Issues (75% of Cases)

Insufficient Water Flow

• Dirty or clogged filter reducing flow
• Partially closed valves
• Pump running on low speed
• Air trapped in system
• Blocked or restricted plumbing

Flow Bypass Problems

• Internal bypass stuck open
• External bypass valve improperly adjusted
• Automatic bypass responding to high restriction

Component Failures

High Limit Switch Issues

• Switch contacts corroded or pitted
• Switch out of calibration (rare)
• Mechanical failure preventing reset

Temperature Sensor Problems

• Sensor reading incorrectly
• Poor thermal contact in well
• Damaged sensor wiring

System Design Problems

• Heater oversized for flow rate
• Multiple heaters with inadequate flow distribution
• Pump and heater mismatch
• Excessive pipe restrictions

Immediate Response Steps

When HS Code Appears

1. Turn off heater immediately - Don't attempt to restart
2. Let system cool - Wait at least 30 minutes
3. Check water flow - Verify pump is running properly
4. Inspect filter - Clean if pressure is high
5. Feel return water - Should not be excessively hot

Detailed Troubleshooting Procedures

Basic Checks (Homeowner Level)

1. Verify System Flow

• Confirm pump is operating at proper speed
• Check filter pressure gauge (clean if needed)
• Feel returns for strong flow
• Look for air bubbles indicating low water or leaks

2. Check Valve Positions

• Ensure all valves are fully open
• Verify solar diverter (if present) isn't restricting flow
• Check that spa/pool valves are in correct position

3. Inspect for Obvious Issues

• Look for kinked or collapsed flexible plumbing
• Check for debris in pump basket
• Verify water level is adequate

Intermediate Troubleshooting

4. Temperature Differential Test

• With heater running, measure inlet and outlet temperatures
• Temperature rise should be:
  • 6-10°F for proper flow
  • >15°F indicates low flow
  • >25°F will trigger high limit

5. Flow Rate Calculation

• Use temperature rise to calculate actual flow
• Formula: GPM = (BTU/hr) ÷ (ΔT × 500)
• Compare to minimum flow requirements for model

6. Bypass Adjustment

• If equipped with manual bypass, close it completely
• This forces all water through heat exchanger
• If problem resolves, flow was marginal

Advanced Diagnostics (Technician Level)

7. High Limit Switch Testing

1. Turn off power to heater
2. Locate high limit switch (usually red button or disc)
3. Test continuity across switch terminals
4. Should show continuity when cool (<105°F)
5. No continuity indicates tripped or failed switch

8. Temperature Sensor Testing

1. Disconnect sensor from control board
2. Measure resistance across sensor leads
3. Compare to temperature/resistance chart:
   • 77°F = ~10,000 ohms
   • 104°F = ~5,300 ohms
   • 140°F = ~2,500 ohms
4. Resistance should change smoothly with temperature

9. Thermal Well Inspection

• Remove sensors from thermal well
• Check for corrosion or scale buildup
• Verify sensors seat fully in well
• Apply thermal compound for better heat transfer

10. Heat Exchanger Inspection

• Check for scale buildup reducing flow
• Look for soot on fire side (indicates combustion issues)
• Inspect for signs of overheating (discoloration)
• Check internal bypass for proper operation

Testing Procedures

Manual Reset Test

1. After heater has cooled completely
2. Press manual reset button on high limit (if equipped)
3. Test continuity to verify reset
4. If won't reset, switch needs replacement

Flow Verification Test

1. Install flow meter in system (if possible)
2. Or use bucket test at equipment drain
3. Measure actual GPM
4. Must meet minimum for heater model

Simulated Operation Test

1. With heater off, run pump for 5 minutes
2. Start heater and monitor temperature rise
3. Watch for rapid temperature increase
4. Shut down if approaching 120°F outlet temp

Solutions by Cause

For Flow Problems

• Clean filter: Backwash or replace cartridges
• Adjust pump speed: Increase RPM on variable speed pumps
• Open valves: Ensure full flow through heater
• Remove restrictions: Clear blockages, replace undersized plumbing
• Fix air leaks: Seal suction side leaks causing cavitation

For Component Failures

• Replace high limit switch: Use exact OEM replacement
• Replace temperature sensor: Match resistance specifications
• Clean thermal well: Remove scale and apply thermal compound
• Replace control board: If diagnostics fail after sensor replacement

For System Design Issues

• Add bypass: Install manual bypass for flow adjustment
• Upgrade pump: Install larger pump for adequate flow
• Replumb system: Increase pipe size or reduce restrictions
• Adjust heater sizing: May need smaller heater for available flow

Replacement Parts

Common parts for HS code repairs:

• High Limit Switch 135°F: HAXXLIM1930
• Temperature Sensor: HAXXSEN1930
• Sensor Well: HAXXWEL1930
• Thermal Compound: Generic heat sink compound
• Control Board: Model-specific part number

Prevention Strategies

• Maintain proper filter maintenance schedule
• Ensure pump delivers adequate flow at all times
• Program variable speed pumps for minimum heater flow
• Install flow switch as additional protection
• Check and clean heat exchanger annually
• Verify bypass adjustment seasonally
• Monitor temperature rise during routine service

Special Considerations

High Altitude Installations

At elevations above 2000 feet:

• Derating may cause longer run times
• Temperature rise may be higher than sea level
• Ensure flow rates compensate for increased temperature rise

Solar Heating Integration

When combined with solar:

• Incoming water may already be warm
• Less temperature rise available before limit
• May need to increase flow rate

Spa/Pool Combinations

Systems serving both spa and pool:

• Spa mode typically has lower flow
• Verify adequate flow in spa configuration
• May need separate flow rates programmed

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